The invention concerns a coaxial magnet configuration for production of a magnetic field in an investigation of volume which is suitable for measurement of magnetic resonance with at least one superconducting solenoid coil or with solenoid coils which are radially nested within each other, wherein the windings of the solenoid coil (s) in a radial region about the axis of the magnet configuration are disposed between r1 and r2 wherein r1>r2.
The magnetic field h(z) as a function of the coordinate z on the axis of a magnet system of this kind can be generally expressed as follows:h(z)=h0+h1z+h2z2+h3z3+h4z4+h5z5+h6z6+
The term h0 thereby represents the desired position independent and therefore homogenous magnetic field. The additional terms with the coefficients h1, h2 . . . describe changes in the magnetic field strength as a function of the spatial coordinate z and are undesirable in magnets for applications in NMR, MRI, or ICR apparatus. A mirror symmetric construction relative to the middle plane leads to the suppression of all terms having uneven indices, at least theoretically. Moreover, it is furthermore desirable to choose the shape of magnetic configuration in such a fashion that the interfering coefficients having even indices, in particular those with small indices, systematically vanished. In a suitable magnet configuration, the expression for the magnetic field h(z) can then be simplified as follows:h(z)=h0+h8z8+terms of higher order.
In this case, an interfering field in the form of a parabola of 8th order overlaps the homogenous magnetic field h0. This interference assumes arbitrarily small values for sufficiently small values of z. In a magnetic configuration of this kind there is characteristic length z0 at which the size of the interference h8z08 has a value dhmax which constitutes a tolerable maximum value for the associated configuration. For NMR apparatuses, these values are typically 1 ppm (part per million), relative to the B0 value. In rotationally symmetric magnetic configurations of this kind, the magnetic field h(z) is not only theoretically homogenous within the above mentioned limit along the axis in the region −z0<z<z0, rather within a spherically shaped region about the symmetry center of radius z0. This region is designated as the investigational volume of the magnet configuration. The larger the size of the desired investigational volume relative to the inside diameter of the portion of the magnetic configuration generating the magnetic field, the higher the required order of the magnetic configuration.
In order to produce stronger magnetic fields, cooled windings made from superconducting wire are preferentially used. The simplest geometrical form for such a magnet coil is a solenoid coil. However, solenoid coils are not suitable for production of homogenous magnetic fields, since this simple geometry has, up to this point, not been capable of compensating for the interfering coefficients h2, h4, . . . . It is therefore conventional in the art to use solenoid coils having one or more notches in order to produce homogenous magnetic fields with such windings. Such notches allow the interfering coefficients h2 and h4 to be compensated for so that the above described configuration can represent a magnet configuration of 6th order. A substantial disadvantage of utilization of magnet coils having notches which, in practice, are fashioned by rings filled up with a solid material, is that the bordering windings made from superconducting wire can undergo a transition into the normally conducting state in response to the large magnetic forces which lead to friction or other relaxation phenomena in the bordering region. This process, designated as a quench, leads to a complete discharge of the superconducting magnet with the energy stored in the magnetic field being turned into heat. As a result thereof, an overheating of the normally conducting regions can occur and, in extreme cases, the magnet coil can be destroyed. This makes, in particular, the production of extremely high magnetic fields difficult or makes the magnetic coil more expensive, since one is forced to limit the current strength through the wires in order to keep the magnetic forces which act on the superconducting wire in the regions bordering a notch sufficiently small. The magnet configuration is consequently provided with a correspondingly greater number of windings of expensive superconducting wire and the entire configuration becomes radially larger, which finally limits the maximum achievable magnetic induction in the investigational volume.
An additional problem with magnet configurations is that they produce highly undesirable magnetic fringe fields in a region of up to 10 m about the magnet configuration. In this region, e.g. magnetic storage media can be erased and magnetic objects can be pulled into the magnet configuration and catapulted therein. The stray field therefore constitutes a danger region.
U.S. Pat. No. 6,507,259 discloses a magnetic configuration of the kind described above having additional radially outwardly disposed superconducting shielding coils in which the current flows in an opposite direction. These shielding coils produce a counter magnetic field to substantially suppress the stray field produced by the magnet configuration (active shielding). However, towards this end, additional windings of superconducting wire are required as well as a support body for the shielding windings as a result of which the magnet configuration becomes more complicated, larger, and more expensive.
Another possibility for suppressing the stray field produced by the magnet configuration is disclosed in U.S. Pat. No. 4,590,428. The collar configuration disclosed therein is surrounded with a ferromagnetic cylinder jacket, which serves for shielding outer stray fields and also effects a feed-back for the magnetic flux of the magnet configuration to thereby limit the magnetic stray field produced by the coil configuration (passive shielding). This configuration has the disadvantage that the weight of the magnet configuration is very high due to the large ferromagnetic cylindrical jacket and, in addition, produces an undesirable influence on the homogeneity of the field.
It is therefore the object of the current invention to propose a simple magnet construction for the production of strong magnetic fields with which there is no need for notches in the solenoid coils of the magnet configuration, wherein the magnetic field produced by this magnet configuration should have high homogeneity in the investigational volume. A further object of the invention is to minimize the magnetic stray field of the magnet configuration in an economical fashion.